Add CI job for testing without msrv-locked Cargo.lock

[matthiasbeyer]

I think just running it on pushed master would be sufficient, wouldn't it?
It still would need to be checked manually from time to time.

Originally posted by @matthiasbeyer in #496 (comment)

We should add a CI job to run the code through tests without the msrv-locked Cargo.lock file.
That job should only run on master commits.